Electron guns provide an effective means for expelling from a body, for example from an orbiting space vehicle, the accumulations of negative charge which may come to be formed on said body either as a result of physical phenomena which have already been studied and verified in practice [Henry Berry Garret, THE CHARGING OF SPACECRAFT SURFACES, Reviews of Geophysics and Space Physics, Vol. 19 No. 4, pages 577-616, November 1981], or as a result of experiments and operations carried out by man [NASA, TETHERS IN SPACE HANDBOOK--Second Edition May 1989].
The acquisition (on the part of an orbiting vehicle) of a negative potential with respect to the surrounding environment (in this case understood as being at zero potential) is in many cases an undesirable circumstance. This causes either the possibility of electrical discharges between vehicle and environment, or between vehicles in the docking phase. The electrical discharge can cause damage and/or disturbances to the structures and/or equipment, with particular reference to the electronic equipment, or the impairment of the preexisting environmental conditions (body at zero potential) which can reduce the meaningfulness of particular scientific experiments.
Other devices (such as a plasma contactor) are in current use in the space field for the limitation of the potential of a vehicle with respect to the surrounding environment (plasma) They emit during their operation a plasma formed of ions, electrons and neutral particles of gas (for example Xe, Ar), and their use is accordingly not practicable in cases in which it is desired to minimize the impairment of the pre-existing environmental conditions.
In these cases, it is advantageous to use an electron gun, which emits only electrons and is moreover adjustable by means of the accelerating voltage. By means of this device, it is in fact possible to expel in a controlled and controllable manner from a space vehicle (or more generally, from a body) negative charges and thus to determine the potential thereof. The negative charges are extracted from the vehicle and injected into the surrounding plasma by means of the emission of an electron beam from the gun, which is electrically connected to thee vehicle in an expedient manner. The electron beam emission must take place when the gun is under appropriate vacuum conditions, and may thus advantageously be used in the space field.
The systems employed up to now (FIG. 1) provide the interposition of a feeder G between the cathode K of the gun and the vehicle C (as exemplified in FIG. 1), for the purpose of bringing the cathode K to and maintaining it at a negative potential with respect to the vehicle C. The accelerating electrode or electrodes E are connected to the potential of the vehicle C. In such a configuration, the feeder G is employed to stabilize a potential difference between accelerating electrodes E and cathode K by accelerating the electrons. The feeder G must supply all the electron current emitted by the cathode K. Moreover, the gun-feeder system must be managed by an external intelligence which controls the times of operation thereof and the modes of application thereof, in order to avoid an uncontrolled expulsion of negative charges. Such an uncontrolled expulsion of negative charge might for example even bring the vehicle to positive potentials with respect to the surrounding environment A, and of a level equal to the accelerating potential difference imposed between vehicle and cathode. Such an event, which has already been experienced in practice, again implies the above cited disadvantages (risk of electrical discharges, impairments of physical parameters of the vehicle-environment system).